Being paid to read Jilly Cooper in the bath is a mixed blessing. Initial yelps of slacker's delight soon give way to the realisation that the obligatory consumption of froth suppresses the appetite for it. Cooper, as has been observed, is well past her golden period, that shimmering 80s bonkbuster reign featuring the true glories of the genre that were Riders, Rivals and The Man Who Made Husbands Jealous. It all started to go wrong around the time of Appassionata, when the frivolously sublime became touched with the ridiculous.

Wicked! is an extremely uneven return to form: baggy, barking mad, but patchily highly addictive, and still blessed with the trademark exuberance and irreverence that wrap the reader in a philanthropic glow. However, this massive, meandering tome needs a thorough seeing-to with a delete button. Wicked! is as long as Anna Karenina and that, surely, is a mistake.

Set in the hazy Cotswolds county of Larkshire, this is a tale of two contrasting educational establishments: a famous public school named Bagley Hall, and a sink comp called Larks that services a heaving estate. Janna Curtis, a red-headed young spitfire from up north, takes on the job of turning round the failing comprehensive. And gosh, readers, what a job it is! Girls named Pearl, Kitten and Kylie Rose mate and brawl with lads called Feral, Monster and Satan who specialise in vandalising, truanting and urinating in phone boxes before progressing to "the dole queue or the nick". Janna is determined to win the hearts and minds of her unwashed flock with a combination of youth, empathy, and a fearsome temper. Pandemonium naturally ensues.

In the meantime, Hengist Brett-Taylor, the married head of the toff school, launches a cunning charm offensive, suggesting a Larks-Bagley partnership that will gain tax concessions for himself and ensure constant contact with the nubile Janna, despite her political objections to the independent sector. Hoorays and oiks proceed to share facilities, school trips and drama departments, providing further opportunities for havoc and copulation.

Paris Alvaston, a gifted Larks pupil in care, falls in love with Janna, but Janna, all small snub nose, freckles and rages, is abjectly entranced by Hengist, who then poaches Paris. Cooper's heroines tend to be old-fashioned, feisty little things, shouty and foot-stampy, who melt at the touch of a married philanderer. Janna, "that adorable crosspatch", is an enjoyable though somewhat predictable protagonist, whereas Paris is fascinatingly damaged and charismatic. He and girl reporter Dora Belvedon, who flogs stories to the tabloids at the age of 11, should surely figure in future Cooper novels. The old anti-hero Rupert Campbell-Black, protagonist of Riders, makes his usual series of amusing cameos.

Wicked!'s groaning cast resembles a production of Les Mis. Even with constant reference to Cooper's helpful character list, which includes a separate guide to featured pets (Cadbury: a "chocolate Labrador"; Loofah: a "delinquent pony"), such a swelling roster adds to the sense of slapstick mayhem. At least half of them need shooting.

Vintage Cooper glimmers through the verbiage: Wicked! is hilarious, daring and sporadically enchanting, but where Riders and Rivals were tightly written and witty, it is often linguistically lazy. Cooper bravely opens herself up to potential ridicule as she does "common", but despite the odd gaffe, this is a valiant and convincing attempt to capture disaffected, deprived youth. A more dated feel hangs around her main characters: women "cat around", and there's even a "swarthy gypsy face" worthy of 40s Enid Blyton.

The novel, naturally, ends happily and satisfyingly after a gargantuan struggle between the forces of corruption and altruism. No one can create this warm, irreverent brand of froth quite like Cooper. If she just goes through it with a machete next time, she'll be back where she belongs.